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CO2 levels highest since Pliocene, but temperatures haven't caught up

July 29, 2013

Year-round ice-free conditions across the surface of the Arctic Ocean could explain why the Earth was substantially warmer during the Pliocene Epoch than it is today, despite similar concentrations of carbon dioxide in the atmosphere, according to new research carried out at the University of Colorado Boulder.

In early May, instruments at the Mauna Loa Observatory in Hawaii marked a new record: The concentration of carbon dioxide climbed to 400 parts per million for the first time in modern history.

The last time researchers believe the carbon dioxide concentration in the atmosphere reached 400 ppm—between 3 and 5 million years ago during the Pliocene—the Earth was about 3.5 to 9 degrees Fahrenheit warmer (2 to 5 degrees Celsius) than it is today. During that time period, trees overtook the tundra, sprouting right to the edges of the Arctic Ocean, and the seas swelled, pushing ocean levels 65 to 80 feet higher.

Audio Clips

Audio Script

July 29, 2013 Jim White

Something that hasn’t happened on Earth since the Pliocene period, some 3 to 5 million years ago, happened in May of this year. That’s when the concentration of carbon dioxide, or CO2, in the atmosphere registered 400 parts per million at the Mauna Loa Observatory in Hawaii. But there’s a noticeable difference between now and the last time CO2 levels were that high, says CU-Boulder geological scientist Jim White.

CUT 1 (42) “The planet was much warmer. It was 15 to 20 degrees warmer in the Arctic. And the sea level, because of Greenland melting and West Antarctica melting, the sea level was 20 meters higher – 60 feet higher. It was a very different planet the last time we had 400 parts per million of CO2.” (:14)

Climate scientists are puzzled by the difference because when they use their climate models to try and recreate conditions during the Pliocene, the result is an Earth that’s cooler than it was in real life.

CUT 2 (120) “If we put 400 parts per million of CO2 in to the atmosphere in our climate models, we can’t make the Pliocene as warm as it was based on the records that we have from fossils and from other things we measure in the environment to measure temperature. (138) So we know it was warm. But the models can’t get it to be that warm. So a fundamental question is: why?”

To answer that question White and his colleagues launched an investigation and the research team decided to force the model to assume that the Arctic was free of ice in the winter as well as the summer during the Pliocene. Climate models, left to run on their own, show ice reforming in the Arctic during the Pliocene winter.

CUT 3 (557) “What happens is water evaporates off the ocean. What that does is that it puts water vapor in the atmosphere and water vapor in the atmosphere does two really important things to warming. (607) One is that it creates a blanket of greenhouse gas. The other thing is when you evaporate water that takes energy and so then that energy is now in the atmosphere and that energy spreads out from the Arctic Ocean and warms up the entire Arctic.” (629)

Could we see that happening in the near future? Probably not, says White,

explaining that since the Earth is made up mostly of water and water takes a long time to warm, it would take some time for the Earth to warm to Pliocene-type levels.

CUT 4 (335) “It will be a while before our planet acts and looks like a 400 parts per million CO2 planet (440) But will that planet be a Pliocene-like planet with ice-free conditions in the wintertime, with trees growing all the way up to the arctic circle, with sea level high enough to drown out Delaware?”

The goal now, says White, is for he and his colleagues to try and understand what types of conditions could bridge the standard model simulations with the simulations in which ice-free conditions in the Arctic are imposed. If they’re successful, computer models would be able to model the transition between a time when ice reformed in the winter to a time when the ocean remained devoid of ice throughout the year.

Such a model also would offer insight into what could happen in our future, says White.